Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks including ethane, propane, naphtha and hydrowax. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into for instance to methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process.
In an OTO process, an oxygenate such as methanol is provided to a reaction zone comprising a suitable conversion catalyst and converted to ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the methanol is converted to higher hydrocarbons including C4+ olefins and paraffins. In order to increase the ethylene and propylene yield of the process, the C4+ olefins may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene. In WO2009/156433, process is described, wherein an oxygenate feedstock is converted in an OTO zone (XTO zone) to an ethylene and propylene product. Higher olefins, i.e. C4+ olefins, produced in the OTO zone are directed to an olefin cracking zone (OC zone). In the olefin cracking zone, part of the higher olefins are converted to additional ethylene and propylene, however substantial amounts of higher olefins remain in the effluent of the olefin cracking zone. After separating the ethylene and propylene from the effluent of the olefin cracking zone, the remaining effluent of the olefin cracking zone is recycled and fed to the inlet of the olefin cracking zone together with the higher olefins stream from the OTO zone. A problem encountered with the process described in WO2009/156433, is the build-up of C4+ paraffins in the recycle to the olefin cracking zone. Paraffins are produced as a side product in the OTO reaction and accumulate in the bottom stream of the depropaniser together with the C4+ olefin fraction during the work-up of both the OTO zone as the olefin cracking zone effluent. The C4+ paraffins are not converted in the olefin cracking zone and therefore remain in the recycle. Due to the small differences in boiling temperature of the olefins and corresponding paraffins, the C4+ paraffins are difficult to separate from the C4+ olefin fraction recycle. To maintain acceptable levels of paraffins in the C4+ olefin fraction recycle it is therefore necessary to with draw part of the C4+ olefin fraction recycle to the olefin cracking zone as a purge stream. Consequently, part of the valuable C4+ olefins are lost as part of the purge stream.
In U.S. Pat. No. 6,049,017, a similar process is described, wherein an oxygenate feedstock is converted in an OTO zone over a SAPO-34 catalyst to an effluent comprising ethylene, propylene, butylenes and paraffins. A stream comprising butylenes and paraffins is separated from the OTO zone effluent and directed to a butylenes cracking zone, wherein butylenes are cracked over a SAPO-34 catalyst. The cracked effluent of the butylenes cracking zone is combined with the effluent of the OTO zone, thereby allowing any remaining butylenes in the cracked effluent to be recycled to the butylenes cracking zone. Also in the process of U.S. Pat. No. 6,049,017, a paraffin content is built-up in the butylenes recycle. Therefore, a purge stream (drag stream) is withdrawn from the process prior to feeding the stream comprising butylenes and paraffins to the butylenes cracking zone. To prevent the loss of butenes in the purge stream, the purge stream is provided to an oligomerisation reactor, wherein n-butenes undergo oligomerisation to higher olefins, i.e. C8, C12, c16 and higher olefins. These higher olefins are subsequently separated from the paraffins in the purge stream and directed to the butylenes cracking zone. A disadvantage of the process of U.S. Pat. No. 6,049,017 is that the cracking of the higher olefins oligomerisation product results in the increased formation of paraffins and aromatics in the butylenes cracking zone. In addition, the cracking of higher olefins in a butylenes cracking zone is much more prone to coke formation resulting in increased catalyst deactivation. Furthermore, the oligomerisation only provides capture of n-butylenes in the purge stream, iso-olefins are not captured, but purged from the process. Consequently, a less than optimal ethylene and propylene yield is achieved.